1. Field of the Invention
The present invention relates to polymers based on polyalkyl (meth)acrylates, for use as bitumen improving agents; and to bitumen modified with such polymers and having improved engineering properties.
2. Background of the Invention
Bitumen, which is a mixture of essentially different hydrocarbons including varying amounts of paraffinic, naphthenic, and aromatic hydrocarbons, has properties which make it useful in many applications such as a component in road surfaces, as a sealing compound, as a coating material or in the form of tar paper or the like, to protect building structures, and as a caulking or waterproofing material or the like, to protect against ground water.
Bitumen has little or no elasticity. The elasticity which a given bitumen possesses depends on the crude oil from which it is produced. Because of its inherent properties, coatings or pavement layers comprised of bitumen are brittle at low temperatures and soft at higher temperatures.
Attempts have been made to incorporate various additives, particularly polymers, into bitumen, particularly in order to improve its rheological properties such that, e.g., a resulting asphalt, viz. a mixture containing the improved bitumen and minerals and used principally in roadbuilding, has sufficient stability and cold flexibility, as well as high fatigue strength under constantly increasing traffic loads. Polymer addition can increase the so-called plasticity range, i.e., the difference between the softening point and the failure point. If the failure point (DIN 52 012) is lowered and the ductility (DIN 52 0I3) is raised, improvement of cold flexibility is indicated. The objective of using polymers as bitumen additives is to alter the viscoelastic properties of bitumen, in particular to extend the plasticity range and to improve the elastic restoration.
In addition to natural and synthetic rubbers, other polymers which have been tested as bitumen improvers are duroplastic resins and thermoplastic resins (Koerner et al, 1977 Plaste und Kautschuk, 24, 475-478). Principal polymers currently used as bitumen additives are styrene/butadiene copolymers (SB), EPDM copolymers, ethylene/vinyl acetate copolymers (EVA), and atactic polypropylene, the latter being used particularly for industrial bitumen. Polymer-modified bitumens available commercially include Carabit .RTM. (SB, supplied by Shell), Olexobit .RTM. (EPDM supplied by BP), and Styrelf .RTM. (SB supplied by Elf).
Other known bitumen improvers are polymers based on (meth)acrylic acid esters, particularly polymers of alkylmethacrylic acid esters (PAMAs). These polymers are still limited in their applicability, despite the fact that they satisfy a number of criteria as additives. The principal drawback which they have is that they do not sufficiently improve the elastic restoration of the finished elastomer-modified bitumen.
U.S. Pat. No. 2,420,082 describes bitumeniferous mixtures comprised of bitumen and 2-5% of polyalkyl methacrylates wherein the alkyl groups have 8-10 carbon atoms. The ductility of the bitumen is improved. Specific molecular weight data are not given.
U.S. Pat. No. 2,848,429 describes bitumeniferous mixtures with improved ductility, obtained by addition of rubber and polyalkyl (meth)acrylates, each in the amount of 0.01-0.5%. The PAMAs have molecular weights of 5,000-500,000.
Ger. AS 12 40 773 and OS 23 31 727 also describe improved bitumeniferous binders for road surfacing, which contain alkyl methacrylate polymers. No detailed or specific information about the polymers is given.
Polyalkyl (meth)acrylates and copolymers of alkyl (meth)acrylates have also been proposed as improvers for bitumen. The polymer described in Ger. OS 25 09 473 is a solid mixed polymer with molecular weights of 50,000-500,000, comprised essentially of monomer units of alkyl (meth)acrylates with alkyl groups containing more than 6 carbon atoms, 3-20 wt. % of acrylic or methacrylic acid, and 3-20 wt. % of basic nitrogen-containing compounds. In Japan OS 52-141,829, stagewise emulsion polymerized additives are described, and in PCT 88/07 067, polyalkyl methacrylates wherein the alkyl groups have 4-20 carbon atoms and with molecular weights of the polymers of c. 5,000-1,000,000 are described which are present in the bitumen in amounts of 0.0 to c. 10 wt. % to form a modified bitumen.
Criteria for candidates as polymeric additives for improvement of the properties of bitumeniferous mixtures are those polymers which improve the oxidation resistance and thermal stability of the polymer. Other important properties of the polymers include ease of incorporation into bitumen, compatibility with different types of bitumen, and stability of the bitumen-polymer mixture.
Known polymeric bitumen additives adequately improve the cold flexibility of bitumeniferous mixtures and the resulting mineral mixtures. The failure point and ductility of such materials are favorably affected by addition of known additives. However, the fatigue strength and elasticity of asphalts obtained with the known bitumeniferous binders are inadequate. These properties are determined by the elastic properties of the bitumen, as indicated, e.g., by severely deformed asphalt road surfaces encountered in practice. A need therefore continues to exist for an improved polymer for admixture with bitumen.